The brazen bull, bronze bull, or Sicilian bull, was a torture and execution device designed in ancient Greece.
Brief History
According to history; a well known attic worker in bronze, Perillos of Athens invented and proposed it to Phalaris, the tyrant of Akragas, Sicily, as a new means of executing criminals.
Perillos invented the apparus hoping to get a handsome reward, but history has it that instead of being rewarded, Perillus became the first person to be placed and tortured inside the Brazen Bull, then thrown off to die in pain.
Source; Wikipedia
History also has it that when the people became tired of Phalaris's mistreatment, he too was placed and killed by torture in the brazen bull.
Structure
The bull, made of bronze, was in the form and size of an actual bull and had an acoustic apparatus that changed screams of suffering victims into the sound of an angered bull. This sound was often a form of entertainment for onlookers.
Source; Acient-Origins
How It Was Used For Torture/Execution
The bull was often used as a method of extracting truth from suspects, and a way of executing offenders. The convicted were placed inside the brazen bull, usually with their tongues cut out first, if condemed, then locked in the device. A fire was set beneath its belly, heating the metal until the person inside roasted to death.
The head of the bull was designed with a complex system of tubes and stops so that the prisoner's screams were converted into sounds like the bellowing of an infuriated bull.
According to legend, when the bull was reopened, the victim's scorched bones "shone like jewels and were made into bracelets."
Reference:
Wikipedia
HowStuffWorks
Acient-Origins
Openculture9